2017 – The Design Flood Frequencies for My Dam Have Changed – Why?
Mark Pearse, Peter Hill
Risk assessments for large dams and the design of upgrades are often dependent on estimates of peak inflows and outflows well beyond those observed in the historic record. The flood frequencies are therefore simulated using rainfall-runoff models and design rainfalls. The recent update of Australian Rainfall and Runoff (ARR) has revised the design rainfalls used to model floods that are of interest to dam owners. This will change the best estimate of flood frequencies for some dams. However, for most dams the impact of revised design rainfalls on flood frequencies is small compared to other factors that can change (independent of dam upgrades). These include model re-calibrations to larger floods, changes to operating procedures that affect the drawdown distribution and improvements in how the joint probabilities of flood causing factors are simulated. In this paper, we look at how the design flood frequencies for some of Australia’s large dams have changed, the reasons for this and then identify five key questions for dam owners to ask to aid assessment of whether the hydrology for a dam should be reviewed
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Papers 2017
2017 – A Turkey’s Nest Dam on Top of a Waste Rock Dump – An Innovative Solution for the Kidston Pumped Storage Project
Learn moreRichard Herweynen, Jamie Campbell, Mohsen Moeini
Hydropower storage plays an expanding role in integrated power systems internationally and can enable increased use of intermittent renewable energy sources such as wind and solar.With an increased amount of renewable energy within the Australian grid, pumped storage has gained increased focus in the past 2years. Entura have been working with Genex Power Ltd. to investigate, evaluate, optimise and design the Kidston Pumped Storage Project, located at the old Kidston gold mine in Northern Queensland. Through this design process, the final arrangement developed included an upper reservoir turkey’s nest dam to be built on the existing waste rock dump on the northern side of the Eldridge Pit, using the existing waste rock dump material and lining it with an HDPE liner. The original waste rock dump was formed during the mining operation by progressively dumping the waste rock predominantly from the Eldridge Pit excavation, with the haul truck traffic being the only compaction that occurred. Since the closure of the mine about 20 years ago, some consolidation of the waste rock dump has occurred.As a result, the key risks identified for the construction of the turkey’s nest dam on top of the waste rock dump were: (1) the stability of the slopes of the waste rock dump, which were generally at the angle of repose for the rockfill material; (2) the absolute settlement of the waste rock dump as the final dam crest level requires a settlement allowance in excess of the flood freeboard requirements; and (3) the differential settlement as excess differential settlement could cause fatigue stress cracking within the liner.This paper presents the investigation and modelling undertaken to confirm the feasibility of constructing this turkey’s nest dam on top of the existing rock waste dump, utilising the historical data on dumped rockfill dams. The paper also presents the feasibility design developed for the upper storage.
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Papers 2017
2017 – Decommissioning of Roaring Meg Dam and Cutten Street Dam No 3 – Creating a New Legacy
Learn morePaul Southcott,Suraj Neupane and David Krushka
TasWater owns and operates the water supply in Queenstown on the west coast of Tasmania. Anew water treatment plant was constructed downstream from one of the seven small dams that made up the original supply system, making the remaining six dams redundant.Two of these dams hada very high annual probability of failure and unacceptable societal life and financial risk due to their poor condition.Both dams required urgent attention (upgrade) to retain them as a lasting asset and legacy for the community or decommissioning to create a new ecological legacy.Roaring Meg Dam (6m high with a 9ML capacity) was constructed on Roaring Meg Creek around 1963. The Cutten Street Dam No 3 (10m high with a 2.4ML capacity) was constructed on Reservoir Creek around 1902to supply water to a growing mining community and had been in use since then. From a heritage perspective, the dam had some value as a timber crib and rockfill dam and its historical context as a key factor in the development of the town.There is limited guidance in the ANCOLD (2003) Dam Safety Management Guidelines on decommissioning and a process had to be developed in cooperation with the Regulator in this relatively new area of dam engineering. Detailed design of the decommissioning including diversion work during decommissioning, channel design to align with the original creek to help restore its ecological function and rehabilitation work on the exposed reservoir soils to stabilise them were undertaken. Aboriginal and historic heritage studies, flora & fauna studies and fluvio-geomorphological study at the dam sites were also undertaken to ensure that the decommissioning work did not interfere with the heritage, threatened species and riparian processes. The community were consulted to ensure acceptance of the changes to their town. Dam safety emergency management plans for the decommissioning of these dams was were also prepared. A significant issue in the decommissioning work was frequent and high rainfall due to the location of these dams on the west coast of Tasmania. The entire dam removal work had to be planned within the window of dry weather or very little rainfall. This paper presents the process, activities and lessons learned in successfully decommissioning these dams,to eliminate the unacceptably high risks posed by these dams and to restore the normal riparian processes.The general approach adopted for this project has applicability for other damsandis proposed as a starting point for an ANCOLD practice note in this area
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Papers 2017
2017 – The Legacy and Potential of the Junction Reefs Dam
Learn moreSteven E Pells, Philip J N Pells
Junction reefs dam was designed in 1895 and constructed by 1897 as a multiple arch brick structure which was the first of its kind in Australia, and one of the earliest in the world. The dam was envisioned to provide mechanical and electrical power for gold mining. This paper provides an historical overview of the unique structure, and reassesses some of its engineering characteristics, such as the stress conditions in its unusual arches and reverse concrete gravity wing walls. The hydrology of the dam is re-assessed from the viewpoint of evaluating its potential as a mini hydro scheme. Commentary is also provided on the performance of its unlined spillway, which has been subject to regular spills for 120 years.
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Papers 2017
2017 – Tailings Dam Failures – Lessons Learned and Some Recommendations
Learn moreHongyu Li
Although the total tailings dam failure frequency peaked in 1960s through 1980s, the failure rate of significant tailings dams has not dropped. The significant tailings dam failures the mining industry experienced in the recent history include: Merriespruit, South Africa, 1994; Los Frails, Spain, 1998; Kolontár, Hungry, 2010; Mount Polley, Canada, 2014; and Samarco, Brazil, 2015. The dam failures may be due to inadequate design, poor construction and inappropriate operations.This paper discusses the lessons learned and some recommendations and good practices to reduce the tailings dam failure risks. It addresses existing issues and provides some recommendations in risk based design, water management-integrity of facilities and water balance modelling, loading rates, tailings farming, adequate governance and roles and responsibilities of designers and nominated engineer.
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Papers 2017
2017 – Comparing the New Rare Design Rainfalls with CRCFORGE Estimates
Learn moreC.Jolly and J.Green
New rare design rainfalls were released for Australia in February 2017, for durations from one to seven days and probabilities from 1in 100Annual Exceedance Probability (AEP) up to 1 in 2000 AEP.The differences between the previous rare design rainfalls using estimated Cooperative Research Centre –FOcussed Rainfall Growth Estimation (CRC-FORGE) method and the new rare design rainfall estimates vary with location, duration and probability. In this paper, these differences are explored spatially through the use of national maps, comparing percentage change between the two datasets for selected durations and probabilities. Before this comparison with the new rare design rainfalls could be completed, the State-basedestimates had to be resampled and aggregated to form a national data set for Australia.For rare design rainfalls, it is often the catchment values that are required to determine the gross rainfall for design purposes. The impact of the revised areal reductions factors and rare design rainfalls is explored through case study catchments in Tasmania.
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